In the first criminal charge to result from the sweeping federal probe into the 2010 BP oil spill, a former BP engineer was arrested by FBI agents Tuesday on charges of obstruction of justice. He allegedly deleted hundreds of text messages from his iPhone that estimated the rate at which oil was spewing from the busted well.

BP PLC/The Associated PressDrilling mud escapes from the broken pipe on the gushing oil well in the Gulf of Mexico. A former BP engineer who assisted in attempts to stop the flow of oil from the company's Macondo well after the Deepwater Horizon explosion was arrested Tuesday on charges of intentionally destroying evidence concerning the amount of oil released from the well.

Kurt Mix, who resigned from BP in January, was charged in a criminal complaint filed Monday in federal court in New Orleans. The complaint, supported by an affidavit from FBI Special Agent Barbara O'Donnell, was unsealed Tuesday.

Mix, 50, of Katy, Texas, assisted in efforts to stop the flow of oil after the April 20 explosion that sank the Deepwater Horizon and killed 11 workers. If convicted, Mix could face up to 20 years in prison for each count.

The criminal charges might help the Justice Department build a foundation for the multibillion-dollar claims it's making against BP in a related civil case. Fines will be based on the amount of oil spilled, and scientific estimates of the flow rate provide the government's basis for alleging 4.9 million barrels came out of the well. BP disputes those figures, but it has been fighting in civil court to keep internal documents about the flow rate secret. The criminal charges against Mix served notice that the government will use BP's own estimates against the company.

And O'Donnell's affidavit supporting the charges strongly suggests that BP knew more oil was coming out of the well than it was reporting to federal agents and the public, which could lead to charges that company officials made false statements to federal authorities.

'Legal hold' on messages

Starting the day after the explosion, Mix had been modeling the flow rate from the leaking well. The number was a source of great controversy in the early days of the spill, when BP was publicly estimating flow rates that turned out to be gross underestimates. Mix shared wide-ranging estimates -- most of which were far above the 5,000 barrels the Coast Guard was then estimating -- with an unnamed supervisor.

The next day, Mix received a "legal hold" notice from federal investigators that warned him not to withhold or delete any evidence. He received five similar notices during the next two months, according to the affidavit.

The charges against him center on text messages Mix sent to BP officials and contractors describing BP's highly publicized attempt to plug the out-of-control well over three days in late May with a mixture of drilling mud and "junk" -- rubber balls and pieces of plastic -- in a procedure called "top kill." Mix later destroyed those texts, the complaint states.

"Both BP's CEO and COO at the time publicly stated (on May 24) that Top Kill had a 60 to 70 percent chance of success," O'Donnell's affidavit notes. Their statements came about a week after a team that included Mix and the unnamed supervisor determined top kill likely wouldn't work if the oil was flowing at more than 15,000 barrels per day.

BP Chief Operating Officer Doug Suttles has said repeatedly that knowing exactly how much oil was flowing was irrelevant to efforts to stop or contain the well.

Late on the first night of top kill, according to O'Donnell, Mix sent a text to a supervisor that said in part: "Too much flowrate -- over 15,000 and too large an orifice."

Nonetheless, two days later, the affidavit says, "BP continued publicly to state that Top Kill was broadly proceeding according to plan."

The next day, BP announced top kill had failed.

Financial considerations

On the next trading day, June 1, the company's stock price dropped by 15 percent, the affidavit noted. That same day, Attorney General Eric Holder announced a federal criminal investigation into the spill. In August, the Securities and Exchange Commission submitted a subpoena to BP for all documents between April 20 and Aug. 9 related to the well's flow rate.

One part of the Justice Department's inquiry has been examining whether BP officials were engaged in securities fraud.

"The charges brought today could be a first step toward building a case that BP officials were understating the size of the spill in an effort to protect the company's stock prices, but it would take far more evidence than got presented today to even consider bringing those charges," David Uhlmann, a University of Michigan environmental law professor, said Tuesday. Uhlmann is also a former chief of the Justice Department's environmental crimes section.

Mix allegedly started deleting messages after he was asked to preserve them and turn them over as part of the investigation.

"Most of these texts ... have been recovered using forensic tools, but some are still unrecoverable," O'Donnell said in the affidavit.

Joan McPhee, an attorney for Mix, called the charges against her client "misguided."

"The government says he intentionally deleted text messages from his phone, but the content of those messages still resides in thousands of emails, text messages and other documents that he saved," McPhee said. "Indeed, the emails that Kurt preserved include the very ones highlighted by the government.

"These misguided charges over failure to retain text messages constitute startling government overreaching. We have every confidence that Kurt will be exonerated at trial."

Crucial number to determine

The question of flow rate has long been contentious, both because investigators are keen to determine whether BP officials lied to the government early in the response, and because the fines BP and other responsible parties are forced to pay will be based on the amount of oil that was released. The Clean Water Act allows for penalties of $1,100 per barrel, and up to $4,300 if the discharge is the result of gross negligence.

Based on federal estimates of the spill, fines could total up to $17.6 billion.

The argument about how much oil was being spilled began within days of the accident, with federal officials complaining about the access BP provided to the wellhead and the quality of videos of oil spewing from the well.

The argument continues today in federal court, where U.S. District Judge Carl Barbier will preside over a key hearing to discuss the future of a bundle of civil lawsuits against BP, including the government's civil case. Less than two hours after Mix's arrest, Justice Department attorney Thomas Benson filed papers in that case objecting to BP's failure to produce details of evidence the company contends is privileged, including information on the flow rate.

"It appears that BP intends to argue that the United States internal flowrate work should be produced while BP's should be protected," Benson wrote. "That position is neither fair nor grounded in the law."

It's unclear whether the documents Benson referenced included any of the text messages that Mix allegedly deleted.

Investigations continue

Tulane law professor Ed Sherman said if the charges against Mix lead to proof of similar obstructions by other BP officials or BP contractors, it could result in sanctions against BP that would make it more difficult for the company to defend itself in the civil case.

Uhlmann said "the question of whether BP misled the government and the general public" is "an appropriate area for the government to focus, but it shouldn't do so to the exclusion of also addressing the spill itself, and the tragic loss of life that occurred on the Deepwater Horizon."

A focus on those areas could lead to criminal charges under the Clean Water Act and the possibility of manslaughter charges, he said.

In a prepared statement, Holder said the investigation into the spill is continuing.

Curtis Thomas, spokesman for BP's Gulf Coast Restoration Office, said the company is cooperating with the various investigations, but he would not provide any information about Mix's role with the company. If Mix deleted any messages, he did so on his own, Thomas said.

"BP had clear policies requiring preservation of evidence in this case and has undertaken substantial and ongoing efforts to preserve evidence," he said.